An immediate allergic reaction to the first dose of an mRNA COVID-19 vaccine - those manufactured by Moderna and Pfizer BioNTech - should not keep people from getting the second dose, a multi-hospital analysis shows.

The study, published today in JAMA Internal Medicine, showed that individuals who had an allergic reaction to the first dose of an mRNA vaccine went on to receive the second dose and tolerated it without complications.

The most frequently reported first-dose reactions were symptoms such as itching, flushing or hives.

The research was led by allergists at Massachusetts General Hospital (MGH). First authors are Matthew Krantz, MD, a second-year clinical fellow in Vanderbilt University Medical Center's Allergy Immunology Fellowship Program, and Jason Kwah, MD, MSCI, of Yale School of Medicine. Other VUMC authors were Cosby Stone Jr., MD, MPH, and Elizabeth Phillips, MD.

With the initial rollout of mRNA vaccines for COVID-19, there was concern about patients with underlying allergic histories, Krantz said. Allergic reactions after receiving an mRNA COVID-19 vaccine have been reported to be as high as 2% with anaphylaxis, a life-threatening allergic reaction occurring in up to 2.5 per 10,000 people.

The investigators from MGH, VUMC, University of Texas Southwestern Medical Center and Yale School of Medicine looked at data from patients who sought care from an allergy specialist after having a reaction to their first dose of an mRNA vaccine, Krantz said.

Among 189 patients, 32 (17%) experienced anaphylaxis after their first dose of the vaccine. Of those, 159 (84%) went on to receive a second dose, and all 159 patients, including 19 who had experienced anaphylaxis following the first dose, tolerated the second dose, Krantz said. Only 32 (20%) of patients reported potential mild allergic symptoms immediately after the second dose. These mild symptoms were resolved with antihistamines, he said.

An important finding from the study is that the reactions to mRNA vaccines may not be related mechanistically to classical allergy, which is mediated by IgE (immunoglobulin-E antibodies). Compared to classical allergy where re-exposure to the same allergen causes similar or even worse symptoms, symptoms appear to be absent or milder after a second dose of an mRNA vaccine.

"The study provides reassurance that a first-dose potentially allergic reaction shouldn't keep people from getting a second dose," Krantz said, adding that allergy specialists may be useful to help determine whether it's safe to proceed to the second dose and complete the two-dose vaccination series.

The study was funded by the National Institutes of Health and MGH.